Homebrew Sinker EDM with 3 axis control

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12 Jun 2023 14:46 #273396 by rft
Hello everybody,

I'm building a Sinker EDM from scratch, because I find it very interesting and there isn't really a open source project to build a sinker EDM machine. I don't have any experience with LinuxCNC, but it seemed like the best solution.
The main specs:
  • Mesa 7i96s Card (a card with more pins would be better, but my budget is limited)
  • EDM powersupply is of my own design (BAXEDM is the cheapest commercial solution, but still 3k$)
  • homebuild machine base and motion system, mostly stuff of ebay
  • custom flushing system (water based, because petroleum is a fire hazard and I want the machine to run unsupervised )
  • fairly small build volume of 100x100x140mm
  • renishaw probe
  • nice control panel
The most interesting and challenging part is the EDM power supply. I want a modern, solid-state pulse power supply, which is controllable via ethernet from LCNC. The burn current, ignition voltage, burn voltage, ON time, OFF time and duty cycle will be controllable.
The idea is to analyze every discharge, in order to detect "bad discharges", that cause excessive electrode wear and turn the pulse off early.
The information that can be found online of the topic of EDM power supply design is very sparse and not practical.
The mechanical design is fairly straight forward, like a C type milling machine with 3 axis. The X-Y stage is most likely from a microscope. The forces in sinker EDM a minimal, so the mechanical stuff won't give me much issues.
The flushing system is based on a particle filter for wire EDM and a de-ionisation filter. Total system water volume is around 30-40l of deionized water.
For the control of the machine I will use is the adaptive feed control feature in LCNC, it is exactly what I need. Foward, stop and reverse the tool path at a adjustable feed rate. Anybody have experience with this feature?

current progress of the project:
  • CNC control - 70% done
  • mechanical - 80% done
  • EDM power supply - only exists on paper
  • flushing system - all the parts, except the pumps, arrived
  • control panel - 70% done
  • custom control software - 0% done¬†
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The following user(s) said Thank You: Henk, HenkvE, smc.collins

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12 Jun 2023 19:08 - 12 Jun 2023 19:08 #273414 by smc.collins
I am very interested in EDM both wire and sinker. I think the EDM power supply could be controlled with a launchpadXL texas instruments motor development card. I could be convinced to help write some hal components to make this work, if someone else can handle the electronics design, I have to many projects going right now to dive into this.
Last edit: 12 Jun 2023 19:08 by smc.collins.

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12 Jun 2023 21:00 #273418 by tommylight
What are the voltages and currents used for each?
I would start with a BST7960, it can handle quite some power, PWM with 5V, not much voltage, cheap, 2 bridges each, can be set as active low or active high, can be used as a full bridge to give AC out, etc, etc.
I used it for heatbed and hotend on a 3D printer, stayed cool driving 12A and 3A on the positive side.

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13 Jun 2023 11:26 #273474 by rft
Hello,

the EDM power supply is a pulsed DC constant current type. The requirements are:
  • ignition voltage (in the diagram ui) is adjustable, 30-120V, almost no current. This voltage dictates the distance between electrode and workpiece, at which the spark ignites.
  • burn voltage (ue in the diagram), 10-50V, adjustable, this is where the current flows and metal is being removed. For wire EDM this voltage can even be just a few Volts. Here we need the constant current regulation, because the discharge channel is very low resistance and too much current flowing is going to cause damage to the electrode.
  • frequency of the impulse, few kHz to 500 kHz
  • Duty cycle 10% to 90%
  • current mostly determines the metal removal rate (MMR), the problem is that the higher the current, the more heat is going in the water, so the heat will need to be removed from the system. Right now I think 10A is enough current.
I plan on using the OPA549 high power OP-Amp for the switching and constant current regulation. This chip is a bit expensive, but it offers the switching and current regulation in one package. The micro controller is a RP2040 (raspberry pi pico), it's a normal 32bit ARM Cortex M0+ MCU.
Basically the EDM power supply is going to be linear power supply. Maybe in later iterations a switch mode power supply (like the BAXEDM), but designing a switchmode power supply for EDM is a lot of work.

The main difference between wire EDM and sinker EDM, is that with wire EDM the electrode wear is not a problem at all. The electrode (the wire) is constantly being fed new and unused from the spool.
With sinker EDM, electrode wear will cause a difference between the desired shape (of the electrode) and the actual resulting shape on the work piece. Electrode wear is why sinker EDM machines use many electrodes, that are switched during machining, to spread the wear across many electrodes and to get very close to the desired shape.

The goal of my EDM power supply is to reduce electrode wear as much as possible. The whole EDM machine should be able to to sinker EDM in 3 axis paths, not just in Z. I'm building a sinker EDM, but I think most of the system should be usable for a wire EDM machine.


I will do a write up on the basic principles of EDM in the future, so everybody can quickly understand the process. The information online is quite insufficient, I have been researching EDM for some time now and collecting information. The best source I have found on how a EDM power supply works is a German PhD thesis. I have it scanned, but it's still in German.

 
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The following user(s) said Thank You: tommylight, Clive S, HenkvE

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13 Jun 2023 21:50 - 13 Jun 2023 21:51 #273516 by smc.collins
a RCS rectified linear psu isn't that difficult to implement, there are tons and tons of good debugged reference designs. actually this sounds like a job for a igbt inverter welder psu, and those are available from multiple sources
Last edit: 13 Jun 2023 21:51 by smc.collins.

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06 Dec 2023 12:04 #287349 by HenkvE
Hi rft
Has there been any progress on your project with the Sinker EDM?

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